Organic light-emitting devices (OLEDs) are now being commercialized for use in flat-panel displays and as solid-state lighting sources. The internal quantum efficiency of some state-of-the-art OLEDs can be nearly 100%. However, due to the refractive indices of the organic layers and the substrate being higher than the refractive index of air, the light generated in the organic emissive region can be emitted into three modes as shown in FIG. 1. These three modes include: (i) external modes, which can escape through the substrate; (ii) substrate-waveguiding modes, which extend from the substrate/air interface to the metal cathode; and (iii) ITO/organic-waveguiding modes, which are confined in the ITO (transparent anode) and organic layers. Typically, only about 20% of the energy is contained in the external modes, suggesting a very low light outcoupling efficiency.
Microlens arrays at the substrate/air interface have been used to effectively extract the substrate-waveguiding modes, leading to a reported 50% improvement in the light outcoupling efficiency. Calculations based on ray-optics show that the maximum outcoupling efficiency using this method can be up to 45% when hemispherical microlenses whose refractive index matches that of the substrate are used. This method, however, does not have any effect on the ITO/organic-waveguiding modes as these layers are spatially separated from the microlenses by the substrate.
An example of a conventional OLED structure, or “bottom-emission” device, is shown in FIG. 1, and includes a transparent substrate, a transparent anode (ITO), organic layers, and a reflecting metal cathode. Light is emitted through the substrate in this bottom-emission device. “Top-emission” OLEDs have been made, in which a reflecting electrode is deposited on a substrate followed by the organic layers and a transparent electrode on top. Light is emitted through the top transparent electrode in this geometry. There are only two modes of light emission in the top-emission device, which include (i) the external modes and (ii) the organic/transparent-electrode-waveguiding modes. The outcoupling efficiency is only slightly improved over that of the conventional OLEDs as the amount of light contained in the external modes is mostly determined by the contrast of refractive index between the organic layers and the air, which is not changed by the elimination of the substrate-waveguiding mode.